When subterranean petroleum deposits are located, the current field practice comprises penetrating the formation with a well, and providing flow conduit in the well and establishing fluid communication between the inside of the flow conduit and the petroleum-containing layers or intervals of the formation by perforating the well casing, in order to permit the petroleum to flow into the well so it may be pumped to the surface of the earth. Many subterranean, petroleum containing reservoirs are comprised of a plurality of strata or separate earth formations, and it is not unusual for a producing well to be in contact with one or more separate intervals from which petroleum is recovered and also be in contact with one or more intervals from which water is produced from the reservoir. Even in the early stages of primary production, some water-production is not uncommon because of these naturally-occurring earth strata which have water present in the pore spaces thereof. Many petroleum formations overlie and contact an active aquifier, and production of large volumes of water from the underlying Aquifier is a common result in such instances. Even though the intention at the time the well is completed is to perforate the well casing so communication between the well and formation is established only in the intervals where oil production may be expected, some water production is encountered even in the early stages of production. Moreover, the dynamics of production activities sometime results in an increase in the amount of water being produced from certain intervals. Even if the oil-production volume remains constant, the ratio of water to oil often increases with time as production is continued from a well, until the amount of water being produced is too great to allow continued production from the well because of the cost of producing and disposing of large volumes of water recovered from the well.
Another problem frequently encountered in wells including oil wells completed in earth formations concerns leaks which sometimes develop between the well interior and the formation. Oil wells are usually completed with a steel casing lining the well, and holes sometime develop in the casing which allows undesired passage of fluid between the well and the formation. Leaks in casing can result from corrosion, shifts in earth formation, or because of failure in the cement used when installing the casing at the time the well was drilled. Any of these occurrences can result in passage of fluid from the formation into the well, which interferes with oil production, or passage of fluid from the well into earth formations, which can result in loss of oil or pollution of water zones or both.
Modern logging techniques widely available in the field allow accurate determination and location of the zones which are producing oil and zones which are producing water. Many techniques have been attempted in field operations, and used commercially with limited success, for the purpose of reducing the permeability of the zones which are producing water, in order to allow continued production of oil from a well while reducing the volume of water production.
One of the oldest techniques employed for this purpose involves the injection of liquid cement into a permeable formation, followed by shutting in the well for a period of time sufficient to allow the cement to set, thereby rendering the formation so treated substantially fluid impermeable. This same technique has also been applied for the purpose of plugging holes in well casing. While this technique has enjoyed limited success in certain applications, oil field cement cannot be used successfully if the temperature of the formation where the process is being applied exceeds about 200.degree. F. because the high temperature causes dehydration of the cement and results in the formation of a very weak plug. The rapid deterioration of the cement plugged zone, due to high temperature, renders this technique unsuitable for treating high temperature wells. Susceptibility of cement to high pH similarly makes cement plugging unsuitable for use in wells in an oil formation being stimulated by injection of fluids having relatively high pH.
More recently, improved processes have become available employing a plastic or resin treating technique wherein a resinous material is injected into the formation and allowed to set, thereby reducing the permeability of the treated zone. This technique makes use of urea formaldehyde resins, and is more successful than cement but is still not useful when the formation temperature exceeds about 200.degree. F. Urea formaldehyde resins are prone to autocatalysis at high temperatures, and so the set time of the injected resin is so short that the treating fluid cannot efficiently invade the zone where the treatment is to be applied.
Other methods have been proposed for use in treating water-producing intervals, employing two or more separate fluids which are injected sequentially with the hope that they will mix after injection into the formation, causing the formation of a solid material in the flow channels so treated, thus reducing the permeability of the treated portion of the formation. This has not been particularly successful in actual use, however, because it is difficult to achieve the degree of mixing between the two fluids necessary in order to form a uniformly reacted solid material necessary to achieve successful plugging of water-producing intervals.
Another type of process has been employed with limited success using an aqueous solution of a hydrophilic polymer injected sequentially with aqueous solutions of materials which promote cross-linking between the polymers, which gradually increases the amount of polymers absorbed on the mineral surfaces of the formation and ultimately reduces the permeability of the zones. These procedures are expensive and time consuming and have not always produced satisfactory results.
It is apparent from the foregoing discussion that there is still a substantial, unfulfilled need for an inexpensive, simple and reliable procedure which can be applied to formations whose temperatures are greater than 200.degree. F., in order to eliminates the flow of water from water-producing intervals into a well penetrating the formation, or for plugging breaks or holes in well casing.